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Rock Mass Rating to Shear Strength Parameters

Rock Mass Rating to Shear Strength Parameters

Rock Mass Rating to Shear Strength Parameters

Hello All
As part of the new extension to an open cast copper in Zambia, I am involved in a slope design (250m high 40 - 50 deg) in a series of strata ranging from soils and soft rock at the crest to very competent rock at the base.  As part of the design, a limit equilibrium analysis is done, which needs shear strength parameters (cohesion and friction angle).  For the upper strata, no problem with a few triaxial tests.  The lower strata however have been geotechnically mapped wrt ultimate compressive strength (UCS), joint condition and few other bits of information that have been used to calculate (derive?) a rock mass rating (RMR)(Bieniawski, 1973 and Laubscher 1990).  The question is (at last...) how to get the RMR into shear strength parameters for design purposes.  It has been done before with reasonable sucess and any pointers to actual experiences and references would be appreciated.

RE: Rock Mass Rating to Shear Strength Parameters

Hello JonMoon:

Try Rock Slopes, Design, Excavation and Stabilizaion, FHWA-TS-89-045, September 1989. The other text is Rock Engineering by Hoek and Bray. I am not sure of title as someone liked my Ref text better than I did.

I have dealt with rock cuts on Highway slopes and these texts are handy references. I think that you can find what you are looking for in them or they can lead you to relevant references.

Remember, despite how good your design, wear your hard hat
and running shoes when enjoying your majestic slope


RE: Rock Mass Rating to Shear Strength Parameters

The book VAD refers to is "Rock slope engineering", 3rd edition published in 1981 but still a classic reference, a few case histories on open pit mines; a lot more stuff, pretty specific, you can find in "comprehensive rock engineering", Pergamon Press, a huge treatise on rock mechanics.
When needed, I derive strenght parameters (phi and c)from the following equations (phi in degrees, c in KPa):
1) phi=5+RMR
2) c = 5*RMR
both proposed by benjawsky, who also tabulated the strenght parameters in relation to qualitative rock mass ratings (poor, fair, etc.) RMR is meant to be Basic RMR, which disregards coefficients relative to the joints' orientation.
One very important caveat though: before using those parameters you must be sure there is an isotropic condition in the rock mass, that is joints do not organize in sets with specific orientations; this would mean anisotropy, hence the calcs must be done graphically and analitically, where the strenght parameters to be used are those relative to the joints' surfaces, allowing for roughness, distance, soft fillings. All is extensively dealt with in the book above mentioned.I think you should have the stereoplots relative to the slopes, that's SOP in quarries and pit mines projects. One method i used a couple of times is Romana's Slope Mass Rating (SMR), described in "comprehensive rock engineering". In a case like yours it might be used as a second guess, my cases were far less impressive than yours, but quarry designers use it anyhow.
I agree with VAD, in a situation like the one you describe the complexity of the system sometimes may defy the best theoretical considerations.   
Best wishes!

RE: Rock Mass Rating to Shear Strength Parameters

have you considered using Hoek-Brown?  I know it's a bit of a round about way of arriving at Mohr-Coulomb parameters but I've used it in some instances and come pretty close to my perception of the shear strength of the strata concerned.

Your description of the strata could be Nchanga but I presume not since you would have a wealth of data if it was.  (I was there myself some years ago).

K Walton

RE: Rock Mass Rating to Shear Strength Parameters

Thanks guys for your help.  It looks like there are a few ways to skin this particular cat.  What we are doing is having a look at Hoek-Brown using some rocscience software called roclab and comparing previous designs and their RMR's to our current situation.  

Good guess, Mr Walton!  The only problem we have with all the historical data is that the new cut is east of the current ops (almost encroaching on the old River Lode Pit, for which the design information is lost) and the strata are more weathered.

When were you here?  Got some pics if you're interested.

RE: Rock Mass Rating to Shear Strength Parameters

I use Roclab too and it's a great bit of free software.

River Lode goes back a long way, before Prof Jennings & Oskar Steffen were involved and there probably wasn't much design information anyway.

I was Senior Soils Engineer at Nchanga 1975-1979, about the time River Lode was being filled in.  My email address is kwalton@ntlworld.com.  I would love to see some photographs.  Thanks

Kevan Walton

RE: Rock Mass Rating to Shear Strength Parameters

You could also use Galena as your slope stability analysis package. Very useful and you can combine both c' ,phi and Hoek Brown criteria very easily. The new version (4.0 recently available), also includes a probability analysis tool

Gareth Williams
Geotechnical Engineer,
Auckland New Zealand

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